1. Field of the Invention
This invention relates to a drive circuit for vibratory-wave motor.
2. Description of the Related Art
To control the number of revolutions of the vibratory-wave motor, either the drive frequency may be varied with the constant voltage applied to the vibratory-wave motor as shown in FIG. 4, or the voltage to be applied to the vibratory-wave motor may be varied, or these may be combined so that the voltage varies in a number of discrete values with continuous variation of the frequency.
However, the above-described revolution number changeover methods have a drawback that in a case where the number of revolutions is controlled in such a manner that, while maintaining the voltage applied to the vibratory-wave motor at a constant level (for example, 30V), the drive frequency is varied as shown in FIG. 4 (on a curve for 30V of FIG. 4), if as the frequency deviates from the resonance frequency f.sub.30 for the drive voltage of 30V as shown by a curve with the parameter of 30V in FIG. 3, the number of revolution lowers, the current flowing to the vibratory-wave motor will increase, thereby deteriorating the drive efficiency of the motor.
Also, if as shown at 30V, 25V and 20V of FIG. 4, many voltage sources are selectively used in combination with the variation of the frequency for the controlling purpose, it becomes possible to control the speed with the use of such a combination of the voltage and frequency as to produce a relatively good efficiency.
For example, in the state of 30V, the frequency is made variable in a range "a" of FIG. 4 to control the number of revolutions in a range of N.sub.1 -N.sub.2. In a range of N.sub.2 -N.sub.3, the drive voltage is changed over to 25V. With 25V, the frequency is made variable only in a range "b" from a resonance frequency f.sub.25. Further in a range N.sub.3 -N.sub.4, the drive voltage is changed over to 20V. Under the condition of 20V, the frequency is made variable in a range "c" from a resonance frequency f.sub.20. Hence, the rotation speed in a wide range can be realized by varying the frequency which does not very largely deviate from the resonance frequency. Thus, the drive efficiency can be prevented from deteriorating.
However, according to this method, the use of a great number of voltage sources gives rise to a problem.